1. Field of the Invention
The present invention relates to fine hydrophobic metal oxide powder, which is useful in powder coating compositions, toners for electrophotography, cosmetic materials and other powder compositions. When added to powder compositions, the fine hydrophobic metal oxide powder is especially suited for the purpose of, for example, improving powder flowability, preventing caking, and controlling powder electrification. It may be added to liquid resin compositions, rubber compositions and other compositions, as a viscosity increaser, a reinforcing filler or to improve adhesiveness. The invention also relates to a method for producing the hydrophobic fine powder. The invention also relates to a toner composition for electrophotography and for developing various electrostatic images in electrostatic recording, electrostatic printing and the like. The fine hydrophobic metal oxide powder greatly improves the electrification stability even during environmental changes, and also improves the imaging properties and cleaning properties. The invention also relates to a method for producing the toner composition containing the hydrophobic metal oxide powder.
2. Description of the Related Art
In the field of powder compositions, various surface-treated metal oxide powders are prepared by treating the surface of metal oxide powders, such as fine silica, titania or alumina, with organic substances. The surface-treated powders are used as additional agents in toners for electrophotographic appliances such as duplicators, laser printers, common paper facsimiles and others, for improving the powder flowability and the electrification property of toners. In these applications, both the flowability of toners containing the surface treated metal oxide powder and the triboelectric property of the surface-treated metal oxide powder itself (relative to the iron or iron oxide carrier in the toner) are important factors.
In general, a negatively-charged agent is added to negatively-charged toners, and a positively-charged agent is added to positively-charged toners. Metal oxides that are used improve the flowability of positively-charged toners generally have amino groups on their surface, and therefore have a high affinity for water. As a result, the electrification property of positively-charged toners containing such a metal oxide often varies according to environmental changes. In addition, the toners containing the metal oxide undesirably aggregate. It is desirable to minimize both aggregation and environmental-induced changes in electrification.
Various proposals have been made relating to metal oxide powders having amino groups. For example, JP-A 62-52561 discloses treating a vapor-phase process silica with an epoxy group having a silane coupling agent followed by further treatment with an amine. JP-A 58-185405 discloses treating the silica with an amino group having a silane coupling agent and a hydrophobicizing agent. JP-A 63-155155 discloses thermally treating a metal oxide powder with an epoxy-containing, modified silicone oil followed by further treating it with an amino group-containing organic compound.
Regarding such surface-treated metal oxide powders, for example, JP-A2-42452 discloses a technique of dispersing fine silica powder in a high-speed jet stream while the powder is contacted with a treating agent. JP-A 2-287459 discloses a hydrophobic dry-process silica treated with silicone oil or varnish.
Metal oxide powders such as silica and others that are used as thickeners or reinforcing fillers for organic liquids are generally treated with an alkylsilane, an organopolysiloxane or the like, whereby their surface is made hydrophobic. For example, JP-A 51-14900 discloses treating a fine oxide powder with an alkylhalogenosilane; and JP-B 57-2641 discloses a technique of treating fine powder of an oxide with an organopolysiloxane.
To satisfy the increasing need for high-quality images in electrophotography, toners having a smaller grain size are desired. For example, conventional toners having a grain size of 9 .mu.m or so are undesirable, but finer toners having a grain size of 6 .mu.m or so are useful. However, the flowability of the finer toners is poor. In order to improve the flowability, the amount of the agents added thereto is increasing. As a result, the additional agent added to toner begins to have a great influence on the electrification property of the toner. In particular, one serious problem is that the electrification property of the toners containing a large amount of the additional agent often varies according to environmental changes. In addition, the degree of hydrophobicity of the additional agent to be added to toners becomes important.
For these reasons, it is necessary to further reduce the amount of electrification of the additional agent itself.
On the other hand, high-quality imaging requires controllable transferability and cleanability of toners. As a result, the additional agent is required to have good dispersability without forming aggregates.
However, conventional fine metal oxide powders treated with an epoxy group-containing, silane coupling agent or with an amino group-containing, organic compound are poorly dispersable, and, in addition, their hydrophobicity is low. Therefore, adding conventional metal oxide powders to toners is disadvantageous in that the toners will absorb water over time whereby their electrification property will vary and their flowability will be impaired.
When metal oxide powders are treated with an amino group-containing silane coupling agent and a hydrophobicizing agent, a large amount of the amino group-containing silane coupling agent must be added to the powders so that the resulting powders can be non-charged or positively-charged. Even through the hydrophobicizing agent is used for the treatment, the resulting powders are not hydrophobic enough. As a result, adding the thus-treated powders to toners is disadvantageous because the toners absorb water over time whereby their electrification property will vary and their flowability will be reduced. In addition, using the amino group-containing silane coupling agent is disadvantageous because it is expensive.
Therefore, the dispersability and hydrophobicity of fine metal oxide powders treated with an epoxy group-containing modified silicone or an amino group-containing organic compound are not satisfactory. Therefore, adding the powders to toners is disadvantageous because the toners will absorb water over time whereby their electrification property will vary and their flowability will be reduced. In addition, of the related conventional techniques noted above, the method of dispersing fine powder of a metal oxide by the use of a high-speed jet stream while contacting the powder withe a treating agent is extremely expensive, in addition, completely purging the system with an inert gas is difficult and dangerous. Moreover, hydrophobic dry-process silica treated with silicone oil or varnish undesirably results in aggregates.